1. Field of the Invention
The present invention relates to a focus state detecting device of divided-pupil phase difference method, adapted for use in a camera or the like, and more particularly to a technology, in a focus state detecting device capable of distance measurement in plural distance measuring areas in an image field, for enabling independent optical adjustment in each distance measuring area thereby enabling precise distance measurement in all the distance measuring areas.
2. Related Background Art
The focus state detecting device of divided-pupil phase difference system, adapted for use in a camera or the like, is disclosed for example in the Japanese Patent Laid-open Application No. 2-120712. FIG. 8 schematically shows the principal part of the focus state detecting device described in said patent application. Said device is provided, as shown in FIG. 8, with a field mask 31 having a cross-shaped aperture 31-1 at the approximate center of the image field of an unrepresented objective lens and vertically oblong apertures 31-2, 31-3 formed in the lateral areas, a field lens 32, a diaphragm 33, a secondary optical system 34 having four pairs of secondary imaging lenses, and a photosensor 35 having four pairs of sensor arrays.
The device shown in FIG. 8 is so designed as to be capable of distance measurements in three areas in the image field, and, in the central area, the components are provided in so-called cross type arrangement in which the mutual movement of the light amount distributions of two object images on the sensor plane takes place in the vertical and lateral directions. Such arrangement enables distance measurement in the central and lateral areas of the image field, and, particularly in the central area, it allows satisfactory focus state detection even for an object showing change in the light amount distribution only in the horizontal or vertical direction, by means of the cross-shaped sensors 35-1a, 35-1b, 35-1c and 35-1d.
However, in such focus state detecting devices of divided-pupil phase difference system, including those having plural distance measuring areas as explained above, critical is the relative positional precision between the line, connecting the field mask positioned in the vicinity of the primary image plane and the peak point of face of the corresponding re-imaging lens, and the light-receiving area of the photosensor. For example, in a device having a cross-shaped distance measuring area at the center as shown in FIG. 8, the field mask is shaped as a cross as indicated by 31-1, and the light beam transmitted by such cross-shaped aperture is further transmitted by the field lens 32, then defined by the aperture of the diaphragm 33 and focused by the re-imaging lens 34 onto the sensor plane.
FIG. 9 shows the mode of imaging on the sensor plane, wherein reference numbers 35-1a, 35-1b, 35-1c and 35-1d indicate the light-receiving arrays of the photosensor 35, as in FIG. 8, while reference numbers 36-1a, 36-1b, 36-1c and 36-1d indicate image areas of the central aperture 31-1 of the field mask 31. FIG. 9 shows a state in which the field mask 31, the re-imaging lens 34 and the light-receiving arrays of the photosensor 35 are mutually adjusted in an ideal state.
However, for example if the centers of the field mask 31, re-imaging lens 34 and photosensor 35 are mutually aligned but the photosensor 35 alone is angularly displaced in a plane perpendicular to the optical axis, the images of the field mask 31 formed by the re-imaging lens 34 and the sensor arrays of the photosensor 35 are mutually positioned as shown in FIG. 10, wherein proper focus state detection is not possible because the horizontally paired arrays 35-1c, 35-1d or the vertically paired arrays 35-1a, 35-1b used for focus state detection respectively receive different parts of the image of the field mask. In such case, the photosensor 35 has to be adjusted, by rotation, to the positional relationship shown in FIG. 9.
Also if the field mask 31 and the photosensor 35 are mutually aligned but the re-imaging lens 34 is angularly displaced, the images of the aperture 31-1 of the field mask 31 appear as shown in FIG. 11, and, also in this case, the paired sensor arrays respectively receive different parts of the image. It is therefore necessary to adjust, by rotation, the photosensor 35 to the state shown in FIG. 12, whereby the paired sensor arrays receive the same parts of the image.
Thus, if only one distance measuring area is provided in the image field, there can be executed the rotational adjustment of the photosensor in the situation shown in FIG. 10 or 11, regardless of whether the aperture of the field mask is cross-shaped or not. However, if three distance-measuring areas are provided in the image field, and the re-imaging lens and the photosensor are integrally constructed in three optical systems corresponding to said distance-measuring areas, as in the device disclosed in the Japanese Patent Laid-open Application No. 2-120712 and shown in FIG. 8, the above-mentioned rotation of the photosensor can achieve adjustment in one of three distance-measuring areas but not in the other two areas, because the photosensor can no longer be moved further. In the conventional focus state detecting device with plural distance-measuring areas, therefore, it has been impossible to effect optimum optical adjustment for all the distance-measuring areas, and, for this reason, to achieve distance measurement of high precision in each of the plural distance-measuring areas.